1. Field of the Invention
The present invention is related to techniques for controlling the downtilt angle of phased-array antennas, such as those used in the base stations of wireless communication networks.
2. Description of the Related Art
In a conventional wireless communication network, communications with wireless units (e.g., mobile telephones) are supported by base stations, each configured with one or more antennas that provide communication coverage over an area surrounding the base station referred to as the base station cell. A typical base station cell may be divided into (e.g., three) sectors, with different antennas configured to support communications for the different sectors. In order to provide a relatively large cell size, base station antennas are typically configured at a higher height (e.g., on the tops of transmission towers) than the wireless units located within that cell. In order to communicate with wireless units located anywhere within a base station cell, including right next to the base station itself, base station antennas are typically configured with a downtilt angle to xe2x80x9cpointxe2x80x9d the antennas down to provide the appropriate coverage.
One way to configure an antenna with a downtilt angle is to physically mount the antenna pointing at an angle below horizontal. Another way to achieve a downtilt angle is to use a phased-array antenna that can be pointed xe2x80x9celectricallyxe2x80x9d by selecting appropriate phase shifts at the various antenna elements in the array.
FIG. 1 shows a block diagram of a conventional N-element, parallel-fed, fixed-phase, phased-array antenna 100. Antenna 100 comprises a power splitter 102, N phase shifters 104, each phase shifter configured with a corresponding antenna element 106, where the N phase shifters 104 are configured in parallel to power splitter 102. Power splitter 102 receives an RF signal and distributes that RF signal to the N phase shifters 104 (e.g., splitting the signal power equally or in a shaped (e.g., cosine) manner among the different phase shifters). Each phase shifter 104i shifts the phase of its received portion of the RF signal by a particular fixed phase-shift angle xcfx86i and passes the resulting phase-shifted RF signal to its corresponding antenna element 106i, which radiates that phase-shifted portion of the RF signal as a wireless electromagnetic (E-M) signal.
If the phase-shift angles xcfx86 at the N phase shifters 104 are selected appropriately, the resulting composite radiated E-M signal from the entire antenna array will form a uniform wavefront that propagates in a particular direction. As depicted in FIG. 1, to achieve a particular downtilt angle xcex1, the element array of antenna 100 is configured with a progressive phase shift such that the phase-shift angle xcfx86i applied by each phase shifter 104i increases linearly from the first phase shifter 1041 through the Nth phase shifter 104N.
In general, the greater the number of antenna elements in the array, the more accurately and well-defined can be the coverage area (or footprint) of the antenna. This can be very important, especially in applications such as wireless communication systems, where base stations need to be distributed over a geographic area and configured with antennas that provide precise antenna footprints to ensure complete coverage over that geographic area with some overlap in adjacent antenna footprints to support handoffs for mobile wireless units, yet not with too much overlap in order to avoid undesirable interference between the signals of different wireless units.
Although FIG. 1 shows antenna 100 configured to transmit RF signals, antenna 100 can also be configured to receive RF signals, either at the same time as, or instead of, being configured to transmit RF signals, in which case, power splitter 102 (also) functions as a power combiner.
For relatively large downtilt angles and large arrays (e.g., more than four elements), the phase-shift angle xcfx86i for the last few phase shifters 104i, where i=N, Nxe2x88x921, . . . , can become very large. This is not a problem for fixed-angle arrays. However, since the heights of base station antennas may vary from cell to cell, and the sizes of cells may vary from base station to base station, the magnitude of the downtilt angle will also typically vary from cell to cell. Moreover, the desired antenna footprint for a particular base station antenna may also vary over time, for example, as more base stations are configured within an existing covered geographic area. As such, it is not always practical to design base station antenna arrays with a fixed downtilt angle.
FIG. 2 shows a block diagram of a conventional N-element, parallel-fed, variable-phase, phased-array antenna 200. Like antenna 100 of FIG. 1, antenna 200 comprises a power splitter 202, N phase shifters 204, each with a corresponding antenna element 206, where the N phase shifters 204 are configured in parallel to power splitter 202. In antenna 200, however, the N phase shifters 204 are configured as part of a phase-shifter assembly 208, which is configured to a motor 210, which is in turn configured to a controller 212.
Controller 212 receives phase control signals that determine how to control the operations of motor 210, which in turn drives phase-shifter assembly 208. Phase-shifter assembly 208 is typically a mechanical device with movable components (as driven by motor 210) whose movements affect the electro-magnetic characteristics (e.g., line length) of the various phase shifters 204 to change the magnitude of the phase-shift angle xcfx86i applied by each phase shifter 204i in a controlled manner.
Because the downtilt angle can be varied in a controllable manner, a single antenna design can be used for different base stations having different antenna heights that require different and varying downtilt angles. One advantage of parallel-fed, variable-phase antennas, such as antenna 200, is that they can be implemented with minimum insertion phase (i.e., phase difference) between adjacent antenna elements. For example, if the progressive phase shift needs to be 17 degrees in order to achieve a downtilt angle xcex1 of 4 degrees, then this can be achieved using parallel-fed phase shifters, where the difference in phase-shift angle xcfx86 between adjacent antenna elements 206i and 206i+1 is simply (xcfx86i+1xe2x88x92xcfx86i)=17xc2x0.
Because the insertion phase can be minimized, parallel-fed, phased-array antennas can have relatively wide bandwidths. Typical wireless communication networks use different frequency bands for uplink (i.e., wireless unit to base station) and downlink (i.e., base station to wireless unit) communications. If the bandwidth of parallel-fed, phased-array antennas can be large enough, a single antenna array may be able to support both the uplink and downlink frequency bands. In that case, a single phased-array antenna can be used to both transmit downlink signals to the wireless units and receive uplink signals from the wireless units.
Unfortunately, for large ranges in downtilt angle (e.g., greater than 4 degrees) and large arrays (e.g., more than eight elements), the last few phase shifters (e.g., 204N, 204Nxe2x88x921, . . .) of parallel-fed antenna 200 can become impractical to realize, because those phase shifters must be able to provide a relatively large range of phase-shift angles xcfx86 (e.g., from as small as 0 degrees for a zero downtilt angle to as large as 180 degrees for a downtilt angle of 4 degrees). In order to avoid this problem, series-fed phased-array antennas are typically used.
FIG. 3 shows a block diagram of a conventional N-element, series-fed, variable-phase, phased-array antenna 300. Like antenna 200 of FIG. 2, antenna 300 comprises a power splitter 302, a phase-shifter assembly 308 with N phase shifters 304, each with a corresponding antenna element 306, a motor 310 that drives phase-shifter assembly 308 and a controller 312 that controls motor 310. Unlike antenna 200, however, the N phase shifters 304 in phase-shifter assembly 308 are configured in series with (Nxe2x88x921) power couplers 314 within a power-splitter assembly 302. As indicated in FIG. 3, the outgoing RF signal received by power-splitter assembly 302 is split by the first coupler 3141 into two RF signals: one of which is phase-shifted by the first phase shifter 3041 by a phase-shift angle xcfx861 for radiation by the first antenna element 3061 and the other of which is transmitted to the second phase shifter 3042, which applies a phase-shift angle xcfx862. In a typical implementation where phase-shift angle xcfx861 is always zero, phase shifter 3041 can be omitted. The phase-shifted RF signal from phase shifter 3042 is then further split by the second coupler 3142 into two RF signals: one of which is transmitted by the second antenna element 3062 and the other of which is transmitted to the third phase shifter 3043, which applies a further phase-shift angle xcfx863 to the already phase-shifted RF signal. The phase-shifted RF signal from phase shifter 3043 is then further split by the third coupler 3143 into two RF signals: one of which is transmitted by the third antenna element 3063 and the other of which is transmitted to the fourth phase shifter (not shown), which applies a fourth phase-shift angle xcfx864 to the twice phase-shifted RF signal. Since phase-shift angles are additive, the RF signal radiated by the third antenna element 3063 has a total phase shift equal to the sum of the phase-shift angles applied by the second and third phase shifters 3042 and 3043 or (xcfx862+xcfx863).
Similar power splitting and phase shifting is repeated for each antenna element until the last coupler 314Nxe2x88x921 is reached. Coupler 314Nxe2x88x921 splits its received RF signal into two RF signals: one of which is transmitted by antenna element 306Nxe2x88x921 with a total phase shift of (xcfx862+xcfx863+. . . +xcfx86Nxe2x88x921) and the other of which is transmitted to the last phase shifter 304N, which applies a final phase-shift angle xcfx86N to the already multiply phase-shifted RF signal before passing the resulting RF signal to the last antenna element 306N, whose radiated signal has a total phase shift of (xcfx862+xcfx863+. . . +xcfx86Nxe2x88x921+xcfx86N).
Because the various phase shifters 304 and power couplers 314 are configured in series (rather than in parallel as in antennas 100 and 200) and since phase shifts are additive, each preceding phase shifter in the series only needs to apply a fraction of the overall phase shift for each antenna element 306 to achieve the desired progressive phase shift for the overall antenna array. As a result, a series-fed, variable-phase, phased-array antenna such as antenna 300 can be designed to provide a wide range of downtilt angles, since each phase shifter needs only to provide a fraction of the overall phase range and is therefore more easily realized.
Unfortunately, however, series-fed antenna designs often do not provide minimum insertion phase. For example, to achieve a progressive phase shift of 17 degrees over an antenna array, the difference in phase shift xcfx86 between adjacent antenna elements 306i and 306i+1 may be (xcfx86i+1xe2x88x92xcfx86i)=377xc2x0, where excess phase in the design is padded by 360 degrees. Over the size of the array, this larger insertion phase makes the phase change rate vary faster as a function of frequency, thereby making the array more narrow in bandwidth. For large arrays (e.g., six elements or more), it is very difficult to achieve a bandwidth wide enough to cover both the uplink and downlink frequency bands for conventional wireless communication networks. As a result, two separate antenna arrays may be needed to support communications between a base station and the corresponding wireless units, with one antenna array designed for the uplink frequency band and the other antenna array designed for the downlink frequency band. In order to support both the uplink and the downlink communications for each wireless unit, the footprints of these uplink and downlink antenna arrays need to be the same and, as a result, their respective downtilt angles need to be able to be coordinated to achieve such common coverage areas.
The present invention is directed to an apparatus for simultaneously controlling the downtilt angles of two (or more) different variable-phase phased-array antennas, such as those used for uplink and downlink communications at a base station of a wireless communication network. Because the uplink and downlink frequency bands in typical wireless communication networks are different, for a common downtilt angle, the progressive phase shifts will be different for the uplink and downlink antennas. The present invention preferably takes those differences into account to achieve coordinated control over downtilt angle for the two different antenna arrays.
In one embodiment, the present invention is an apparatus for simultaneously controlling downtilt angles of two or more arrays of antenna elements, comprising (a) for each array, a power splitter and a phase-shifter assembly configured to control the progressive phase shifts between successive elements in the array; (b) a common linkage connected to one or more movable components of each phase-shifter assembly; (c) a common motor configured to the linkage to convert motion of the common motor into motion of the linkage; and (d) a controller configured to control the motion of the common motor, wherein the motion of the common motor causes the motion of the linkage which simultaneously moves the one or more components within each phase-shifter assembly to change the progressive phase shifts between successive elements in the corresponding array, thereby simultaneously changing the downtilt angles of the two or more arrays in a coordinated fashion.
In another embodiment, the present invention is an antenna system for a base station of a wireless communication network, comprising (a) an uplink array of antenna elements; (b) a downlink array of antenna elements; (c) an uplink power-combiner and an uplink phase-shifter assembly configured to control progressive phase shifts between successive array elements in the uplink array; (d) a downlink power-splitter and a downlink phase-shifter assembly configured to control progressive phase shifts between successive array elements in the downlink array; (e) a common linkage connected to one or more movable components of both the uplink and downlink phase-shifter assemblies; (f) a common motor configured to the linkage to convert motion of the common motor into motion of the linkage; and (g) a controller configured to control the motion of the common motor, wherein the motion of the common motor causes the motion of the linkage which simultaneously moves the one or more components within the uplink and downlink power-splitter/phase-shifter assemblies to simultaneously change the progressive phase shifts between successive elements in the uplink and downlink arrays, thereby simultaneously changing the downtilt angles of the uplink and downlink arrays in a coordinated fashion.